Constraining the Size of the Torus of NGC 6418

One of the main components of the unified model of AGN is the circumnuclear torus consisting of dusty molecular gas. It is important to understand the size and structure of the torus, as it often obscures our view of the central engine of the AGN. Although it is not possible to resolve the torus in images obtained with existing single telescopes, it can be studied by reverberation mapping. In this technique, the time variations of the UV/Optical (UVO) emission from the accretion disk can be analyzed with the infrared (IR) response of the torus dust. The time lag between the driving UVO emission and the IR response can be used to constrain the size of the torus. Our group used the Spitzer Space Telescope to observe 12 AGN over a period of 2.5 years during Spitzer cycles 8 and 9, at the wavelengths of 3.6 and 4.5 microns. Here, we present the cross-correlation analysis results for one of the objects, NGC 6418, which exhibited the largest variations seen in our sample. A large optical flare occurs at the beginning of Cycle 9, after which the reverberation lag increases. The increase in the lag is most likely due to an increase in the inner radius of the torus, due to destruction of dust grains in the inner regions by sublimation. The spectral type also appears to have changed from a Seyfert 1.9 before the flare, to a Seyfert 1 after the flare, suggesting that this object is a changing-look AGN.